1. Field of the Invention
This invention relates to nitride based device structures on patterned substrates, such as nitride based light emitting diode (LED) structures on patterned substrates, having enhanced performance.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
The usefulness of gallium nitride (GaN), and its ternary and quaternary compounds incorporating aluminum and indium (AlGaN, InGaN, AlInGaN), has been well established for fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices. These devices are typically grown epitaxially using growth techniques including molecular beam epitaxy (MBE), metalorganic chemical vapor deposition (MOCVD), and hydride vapor phase epitaxy (HVPE).
Nitride based optoelectronic devices began their quick ascent into commercialization with the advent of the use of a thin nucleation layer prior to the deposition of high quality GaN. This technique is employed due to the lack of a native substrate available for GaN growth. Later techniques such as the development of p-type GaN by magnesium doping followed by high temperature anneal also proved vital. However, the development of using InGaN as the active layer for short wavelength devices allowed nitride based Light Emitting Diodes (LEDs) and laser diodes (LDs) to overtake many other research ventures and has now become the dominant material system used for visible light semiconductor applications.
The external quantum efficiency or total efficiency (ηL) of LEDs can be defined by the following equation:ηL=ηintηinjηext where the extraction efficiency, ηext, is defined as the amount of photons extracted, the injection efficiency, ηinj, is defined as the amount of carriers injected into the active region of the device, and the internal quantum efficiency, ηint, is defined as the amount of photons generated in the active region of the device. The internal quantum efficiency of a device can be maximized by reducing the number of non-radiative centers, such as defects and impurities. The internal quantum and injection efficiency of blue nitride based LEDs have already been improved to a high level by optimizing the deposition conditions of the device layers. Therefore, further improvement in external efficiency of a device would require improvement in the extraction efficiency.
The extraction efficiency of nitride based devices grown on sapphire is hampered by the difference in the refractive index between nitride films and sapphire. This refractive difference, in turn, causes internal reflections which can “trap” the light generated in the active region. Therefore, most of the light that is generated propagates through the nitride film and cannot be used as useful light.
One approach to improve light extraction from nitride devices is to use a patterned substrate on which the device is subsequently grown. A patterned substrate is defined as any substrate which has been processed to produce surface features which include (but are not limited to): stripes, semicircles, pyramids, mesas of different shapes, et cetera. The pattern on the substrate aids in extracting the light emission from the active region of the device by the suppression of light interference. Early work of growth on patterned sapphire wafers by Tadatomo et al. initially tried to reduce the dislocation density of the nitride film by growing on patterned grooves or stripes along different crystal growth directions [1]. This was done in order to avoid a two step growth procedure, commonly referred to as Lateral Epitaxial Overgrowth (LEO), which uses a patterned SiO2 stripe deposited atop an as-grown nitride film, in order to reduce the dislocation density of the nitride film grown atop the stripes. The LEO process is cumbersome due to the fact that the wafer must be removed from the reactor in order to deposit the SiO2 stripes and then re-introduced into the reactor for regrowth of nitride films atop the patterned nitride film. Thus, the advantage of growing on a patterned substrate is that the growth can be performed in one deposition step compared to the two steps of the LEO process.
Further improvements of LED devices grown on patterned substrates showed enhanced light extraction by use of various types of pattern designs [2]. These devices exhibited increased output powers and luminous efficiency compared to LED devices grown on non-patterned substrates. However, these devices employed the use of a standard LED structure. A standard LED structure is described as a structure comprising a sapphire or silicon carbide substrate, a buffer made of GaN or AlGaN, an n-contact layer made of GaN doped with silicon, an active layer made of a single quantum well (QW) or multiple quantum well (MQW) containing InGaN, an electron blocking layer made of AlGaN, and a p-contact layer made of GaN doped with magnesium. This device structure was shown to work well at a forward current of 20 mA, with a light emission at 450 nm, and an output power of 10-15 mW.
Although this standard device structure has worked well for non-patterned substrates, this standard structure has exhibited detrimental performance in output power at equivalent drive currents when used with a patterned substrate. Thus, there is a need for improved device structures in order to increase performance of nitride based LEDs deposited on patterned substrates. The present invention addresses this issue by the use of a device structure which includes a nitride based interlayer located adjacent to the active region of the device.
As stated previously, the current technology used in device structures of nitride LEDs on patterned substrates does not employ the use of a nitride based interlayer. This invention allows for the realization of high output power LEDs grown on patterned substrates. Although the use of a nitride interlayer has been shown to enhance the output power of LEDs grown on conventional non-patterned substrates, a scientific consensus on why this occurs has not been reached [3].
The present invention distinguishes itself from above mentioned device designs by the use of a nitride interlayer on a patterned substrate in order to improve the performance of light emitting devices. As a result, there is a need for improved device design structures on patterned substrates, wherein the device structure minimizes the deleterious effects present in conventional light emitting device structures deposited on patterned substrates. The present invention satisfies this need.